Local density augmentation of supercritical water probed by 4,4′-bpyH radical: A pulse radiolysis study

Highlights: • Pulse-radiolysis study of local density augmentation in supercritical water. • 4,4′-bpyH has a stronger solute–solvent effect in water than in organic solvents. • Density fluctuation (Δρ = ρ_local–ρ_bulk) reaches 280 kg m⁻³ at 380 °C/23 MPa. • Δρ maximizes when ρ_bulk is around 120 kg m⁻³. Solvatochromic shift of 4,4′-bpyH in aqueous solutions at elevated temperatures up to supercritical conditions and in various organic solvents with different dielectric constants, is investigated by pulse-radiolysis technique. 4,4′-bpyH shows a stronger solvent–solute interaction in water than in organic solvents, perhaps due to the hydrogen bond between 4,4′-bpyH and water. At 380 °C, local density augmentation, namely ρ_local–ρ_bulk, in supercritical water becomes 280 kg m⁻³ (ρ_bulk = 208 kg m⁻³), and the density enhancement factor is 8.9. Density fluctuation maximizes when ρ_bulk is around 120 kg m⁻³. Density inhomogeneity decreases as temperature rises, but is still remarkable at 400 °C.